US20020153245A1

US20020153245A1 - Flow-through eletrochemical reactor for wastewater treatment

Info

Publication number: US20020153245A1
Application number: US09/782,279
Authority: US
Inventors: Yves Henuset; Joel Fournier
Original assignee: Individual
Current assignee: National Research Council of Canada
Priority date: 2001-02-14
Filing date: 2001-02-14
Publication date: 2002-10-24
Also published as: US6814840B2

Abstract

Disclosed is A flow-through electrochemical reactor comprising a body having an internal chamber, and an inlet port and an outlet port in communication with said internal chamber to permit flow of wastewater therethrough, at least one porous anode arranged in said internal chamber such that the wastewater flowing between said inlet port and said outlet port flows through the pores of said at least one porous anode, said at least one porous anode having activity for the destruction of a target substance, and at least one cathode disposed in the internal chamber to permit an electric current to be established between said at least one cathode and said at least one anode, said electric current reducing the concentration of said target substance in the wastewater flowing through the chamber.

Description

FIELD OF THE INVENTION

This invention relates to wastewater treatment where levels of organic contaminants, such as phenols and related compounds, are to be decreased.

BACKGROUND OF THE INVENTION

Several industrial processes require the use of large quantities of water for their operations. The water may come from natural sources such as rivers or from treated city water. As a consequence of the industrial activities, the used water may become contaminated with organic pollutants beyond permissible, environmentally acceptable limits.

Organic contaminants can be removed to a limited degree by adsorption on activated carbon, ozonation, or a combination of these methods. After use, activated carbon, if filled with contaminants, requires destruction or disposal to a special landfill. In addition, activated carbon is not necessarily selective enough to efficiently absorb the problem compounds, and when the active sites are full, the adsorption capacity goes down to zero. Ozone is a dangerous chemical and it would be preferred if its use could be avoided in wastewater treatment.

Electrochemical treatment of wastewater can reduce the level of organic contaminants by oxidation. Noding (U.S. Pat. No. 4,652,355) discloses an electrochemical reactor in which the anode and cathode in a reaction chamber are in the same plane as the direction of flow of aryl-containing wastewater. This reactor predominantly produces aryl hydroquinones, which are not ideal end products for environmental release.

Similarly, Cole (U.S. Pat. No. 5,531,865) discloses an electrochemical reactor having a cathode and a plurality of sacrificial anodes elongated in a chamber, parallel to the direction of flow of contaminated water. With such a configuration of electrodes, charge density will vary across the cross-section of the reaction chamber, and it is possible that a significant amount of aryl compounds will not contact an anode, and experience sufficient charge density to be oxidized, while flowing through the chamber.

Several patents have issued relating to reactors that, in attempting to optimize the possibility of electrochemical reaction, make available significant electrode surface area by having multiple solid electrodes in various configurations and/or requiring meandering flow of wastewater over the surface the electrodes (for example U.S. Pat. No. 5,549,812 (Witt); U.S. Pat. No. 5,587,057 (Metzler et al.); 5,611,907 (Herbst et al.); U.S. Pat. No. 5,746,904 (Lee); and U.S. Pat. No. 5,928,493 (Morkovsky et al.)). The reactors found in these patents tend to be of relatively complex construction and the flowpath of the wastewater over solid electrodes, in each case, does not guarantee intimate contact with an anode surface.

Sampson et al. (U.S. Pat. No. 5,705,050) discloses a packed bed reactor, which includes an ion exchange material packed between an anode and a cathode. However, ion exchange materials require special handling and specific reactor conditions to tolerate higher back pressures that can occur.

SUMMARY OF THE INVENTION

By using at least one porous anode, the electrochemical reactor of the present invention addresses limitations in known reactors. By directing the flow of wastewater through the pores of at least one porous anode, the reactor disclosed herein provides a high probability that contaminant molecules will experience intimate contact with an anode and thus encounter the necessary current density for oxidation. This advantage is coupled with the relatively simple construction of the reactor and ease of maintenance.

The invention provides an electrochemical reactor (cell) for reducing the concentration of organic compounds, such as aryl compounds, found in wastewater from industrial processes. Breakdown of the organic compounds occurs by oxidation at the anode of the electrochemical reactor.

More specifically, the present invention provides a flow-through electrochemical reactor comprising:

a body having an internal chamber, and an inlet port and an outlet port in communication with said internal chamber to permit flow of wastewater therethrough;

at least one porous anode arranged in said internal chamber such that the wastewater flowing between said inlet port and said outlet port flows through the pores of said at least one porous anode, said at least one porous anode having activity for the destruction of a target substance; and

at least one cathode disposed in the internal chamber to permit an electric current to be established between said at least one cathode and said at least one anode, said electric current reducing the concentration of said target substance in the wastewater flowing through the chamber.

The reactor, when in use, reduces TOC content of industrial wastewater by oxidizing target substances, such as aryl compounds, efficiently. Efficient oxidation minimizes the possibility of competing side reactions. The side reactions are unfavorable since they might produce compounds that are as harmful as, or more harmful than, the compounds to be destroyed.

Thus, the electrochemical reactor can treat a wastewater stream to reduce the concentration of aryl compounds to an environmentally acceptable level. The reactor of the present invention also offers the advantage that it can be installed within an existing piping system.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the present invention will become apparent, to those skilled in the art to which the present invention relates, from reading the following specification with reference to the accompanying drawings, in which: [0016]
FIG. 1 is a schematic diagram of an embodiment of the flow-through reactor of the present invention; [0017]
FIG. 2 is a top view of an electrode of FIG. 1; [0018]
FIG. 3 is a schematic side view of an electrode and holder of FIG. 1; [0019]
FIG. 4 is an image of a titanium foam used as an anode substrate; [0020]
FIG. 5 shows the morphology of an antimony-doped tin dioxide (SnO[0021] ₂) dimentionally-stable anode (DSA) coating;
FIG. 6 is a graph of the efficiency of phenol destruction by an embodiment of an electrochemical reactor of the invention, at current densities of 1.4 (white), 2.8 (black) and 5.6 (cross-hatched) MA/cm[0022] ², using 3D foam anodes of either antimony-doped tin dioxide, platinum or tantalum-doped iridium dioxide; and
FIG. 7 is a graph of the efficiency of destruction of a mixture of m- and p-cresol by an embodiment of an electrochemical reactor of the invention, at current densities of 1.4 (white), 2.8 (black) and 5.6 (cross-hatched) mA/cm[0023] ², using 3D foam anodes of either antimony-doped tin dioxide, platinum or tantalum-doped iridium dioxide.

DETAILED DESCRIPTION OF THE INVENTION

The following description illustrates the manner in which the principles of this invention are applied but is not to be construed as, in any sense, limiting the scope of the invention. [0024]
Referring to the embodiment of FIG. 1, an [0025] electrochemical reactor 1 in accordance with the present invention includes a tubular body 2 having an inlet port 3 and an outlet port 4. The inlet port 3 is retained on the tubular body 2 with a first retaining means (not shown). An inlet O-ring 5 is disposed between the inlet port 3 and the tubular body 2 in a sealing engagement. Similarly, the outlet port 4 is retained on the tubular body 2 with a second retaining means (not shown). An outlet O-ring 6 is disposed between the outlet port 4 and the tubular body 2 in a sealing engagement.
Inside the tubular body are a series of [0026] porous cathodes 7 and anodes 8 in alternating arrangement, each having a contact wire 9 in the form of a screw passing, in a liquid-tight manner, through holes in the wall of the tubular body 2. The screws also serve to secure the cathodes and anodes in place and are further provided with electrical communication to a DC power supply (not shown). Wastewater is introduced into the reactor through an inlet pipe 10 from a reservoir 11. Treated wastewater leaves the reactor through an outlet pipe 12 and is returned to the reservoir 11. A pump 13 is used to move the wastewater through the reactor. The wastewater supply from the reservoir 11 is controlled by a valve 14.
Referring to FIG. 2, a [0027] cathode 7 is shown which includes an circular, 3D foam-type electrode 15 retained in an insulating electrode holder having a top 16 and bottom 18 (see FIG. 1) held together with screws 17. Each electrode holder is sized to provide a snug fit within the tubular body 2 so that essentially all wastewater introduced into the reactor passes through the porous anodes 8 and cathodes 7.
FIG. 3 shows a foam-[0028] type electrode 15 and contact wire 9 between the top 16 and bottom 18 of the electrode holder prior to assembly by screwing the top 16 and bottom 18 together with the screw 17.
In use, the [0029] reactor 1 can be mounted vertically or horizontally. The reactor should be placed in an open recirculation circuit, thus allowing evolved gases, such as carbon dioxide, to escape.
The body of the reactor can have a variety of shapes but preferably is tubular and the internal chamber cylindrical, with a generally circular cross-section. While FIG. 1 shows detachable inlet and [0030] outlet ports 3 and 4, which permit convenient access to the electrodes in the tubular body 2, a unitary construction is also possible. The electrode holder (see FIG. 3) serves as a mechanical device to install electrodes within the electrochemical reactor 1, as well as an electrical insulator. The insulating holder preferably is sized, conveniently in a disc shape, for close-fitting insertion into the internal chamber. The holders can be held in place within the body 2 by screws passing through the wall of the body, or by some other suitable means. It is preferred that the electrical connection is also provided by the screw which can be connected electrically to a suitable power supply external to the reactor. Conveniently, the power supply is a DC supply.
By removing the [0031] inlet port 3 or outlet port 4, or both, the number, and arrangement, of electrodes in the reactor 1 can be conveniently changed. In addition, the electrodes can be removed from the reactor for periodic cleaning. The cleaning process can also be performed in situ and may involve the use of an organic solvent, such as methanol or ethanol, or an alkaline cleaner, with or without current. The current may be inverted if needed. It is preferred that the anode material is platinum and the cathode material is nickel, because a current polarity inversion to clean them will not result in damage to the electrode materials.
The electrodes are preferably stacked in an alternating arrangement, such that an anode is placed next to a cathode and vice-versa (i.e. C/A/C/A/C/A/C/A/C . . . ). As such, the number of anodes and cathodes in the reactor can be varied, from a minimum of one anode and one cathode to many tens of anodes and cathodes. It is preferred that the alternating arrangement begins and ends with a cathode, to ensure optimum activity of the anode at the start and end of the series. More preferably, there are two to ten anodes and three to eleven cathodes, respectively. Conveniently, there are seven cathodes and six anodes. The number of electrodes used depends upon the volume of the solution to be treated and the desired treatment time. Each anode is isolated from each cathode, to avoid a short-circuit. The anodes and cathodes typically are each connected to corresponding bus bars that in turn are connected to a DC power supply. [0032]
The reactor is made from any material that has the necessary mechanical strength for the chosen dimensions of the reactor, and resistance to corrosion by the wastewater stream of interest. Such materials can be glass, polymer-coated stainless steel, reinforced fiberglass or polymer, and the like. [0033]
Preferably, the wastewater is filtered before treatment in the reactor in order to minimize the possibility of blockage of the electrodes with solid materials. The wastewater to be treated flows through the porous electrodes in the reactor, and therefore the liquid can be treated then conducted to a holding tank. While the solution to be treated flows through the reactor, and hence through the electrodes, a DC current passes within the reactor, between the anodes and the cathodes. The pore openings in the foam electrodes allow a free flow, of the wastewater to be treated, with a minimum of flow restriction. [0034]
Depending upon the anode material, a current density that can vary between 0.7 and 70 (mA/cm[0035] ²) is applied, although for phenolic compounds, a current density of about 1.4 mA/cm²is preferred. For wastewater having several target compounds, zones of different current densities can be established within the reactor in order to optimize the destruction of each target compound. The distance between certain electrodes can be selected based on the desired current density at a particular location in the reactor.
The electrolysis (or treatment) time depends upon the initial concentration of the problem compounds and the final concentration desired, as well as the flow rate. This latter variable can be between 1 to 60 liters per minute of reactor capacity, although a flow of about 8 liters per minute is preferred. The dimensions of the electrodes, and the reactor generally, can be varied depending on specific requirements. Electrode diameter conveniently can be up to about 1.5 m. Electrode thickness conveniently can be up to 3 cm, preferably about 0.5 cm for a titanium substrate. [0036]
The wastewater to be treated can circulate for a variable number of cycles through the reactor, or make a single pass, depending upon the level of initial contamination level and final desired (or required) final level and desired (or required) treatment time. Conveniently, the reactor is used at ambient temperature and pressure, although other conditions can be selected as appropriate. [0037]
Wastewater to be treated can come from industrial sources, such as debarking effluent, and pulp and papermaking effluent. Preferred target aryl compounds in such wastewater are phenol and o-, m- and p-cresol. The reactor described herein has the capability of destroying the target compounds even in the presence of other organic compounds, such as butanoic acid, pentanoic acid, hexanoic acid, butanedioic acid, camphor, borneol, linalyl propanoate, furan carboxaldehyde, cyclohexanecarboxylic acid, 2-(2-hydroxy-2-propyl)-5-methyl-cyclohexanol, benzoic acid, 4-hydroxy-benzenepropanoic acid, or inorganic species such as calcium, iron, magnesium, manganese, aluminum, zinc, sodium and potassium. [0038]
The total organic carbon (TOC) level of the wastewater to be treated is preferably less than 7500 ppb, more preferably less than 1500 ppb. [0039]
The Anode [0040]
The anode conveniently should be made from a material that is stable in the wastewater to be treated, and that provides reasonable activity for the destruction of the target compounds. The anode is preferably non-sacrificial. [0041]
The anode typically is constituted by a coated substrate, the substrate preferably being a valve metal, such as tantalum or titanium. Although various anode Isubstrates could be used, such as nickel, stainless steel alloys or other corrosion resistant materials, titanium is preferred. The anode substrate should be in the form of a porous or 3D medium (sponge, foam, felt or mesh). A foam-type is preferred, such as the Astro Met® materials (Astro Met, Cincinnati, Ohio), in a configuration similar to that shown in FIGS. 2 and 3. Each anode should have a pore opening value of up to 40 pores per linear inch (ppi), preferably 20 ppi, to allow liquid flow with minimal resistance. [0042]
When titanium is used as anode substrate, it is preferably first activated through a process that removes the surface oxide layer. Treating the titanium with boiling concentrated hydrochloric acid is one such process. The treated titanium is then quickly coated with the selected anode material. [0043]
The anode is where the electrooxidative processes take place. Destruction of an organic compound by oxidation is a two-fold process:[0044]
H₂O+M→M—OH.+H⁺+e⁻ Step 1.
In [0045] Step 1, the water molecule is split into hydrogen and hydroxyl radicals. The anode (M) serves as a base to the formation of these two species (it acts as an electrocatalyst). The second step involves the oxidation of an organic compound (R):
R+M—OH.→M+RO+H⁺+e⁻ Step 2.
where RO corresponds to the oxidized organic compound. [0046]
This overall reaction competes with the reaction that forms oxygen. Electrochemical efficiency is defined as the ratio between the two main anodic reactions. [0047]

Anode materials were tested for stability and efficiency to destroy organic contaminants in high TOC wastewater. The results of these tests are summarized in Table 1:

TABLE 1


	Efficiency for	Electrochemical
Anode Material	Organic Destruction	Stability

Platinum	Very good	Very good
Tantalum doped	Very good	Poor
Iridium Dioxide
Antimony doped	Good	Very good
Tin Dioxide

Platinum, electrodeposited on a titanium substrate (see FIG. 4), exhibited high efficiency, together with high stability. Platinum was efficient in electrolyzing wastewater contaminated with phenol compounds, and is thus the preferred anode material. A summary of the evaluated efficiencies of the anode materials described in Table 1 is given in FIG. 6 and FIG. 7. [0049]
As well as platinum exemplified above, other metals such as palladium, rhodium, iridium or ruthenium, alone or in alloys with themselves or other suitable metals, can be used as the anode material. [0050]
Antimony-doped tin dioxide (see FIG. 5) coated anodes have been shown to be good at destroying organic compounds. However, it was the least efficient anode material that was tested in the reactor of the invention, most probably due to the presence of numerous other organic species in the wastewater to be treated. [0051]
Although tantalum-doped iridium dioxide-coated anodes showed a very good efficiency for destroying organic compounds, it was found that, over a period of time, the coating tends to spall off the anode substrate. [0052]
Although it is preferred that the tin dioxide and iridium dioxide coatings are doped as described above, they can each generally be doped with a dopant selected from Sb, Ta, F, Cl, Mo, W and Nb, and mixtures thereof, if required. [0053]
Known coating methods can be used to coat the anodes. The invention is augmented when the coating is uniform and homogeneous on the substrate. [0054]
The Cathode [0055]
A cathode is necessary to complete the electrical circuit and allow the electrochemical oxidation process to be possible. The cathode can be formed from a porous or 3D medium (foam, sponge, felt or mesh) and is preferably of a structure similar to that shown in FIGS. 2 and 3. Each porous cathode should have a pore opening value of up to 40 pores per linear inch (ppi), preferably 20 ppi, to allow liquid flow with minimal resistance. The cathode can also have other structures, such as a ring-like structure. [0056]
The cathode material can be nickel, nickel alloys, stainless steel or even titanium, or any other corrosion resistant material. Nickel is preferred because of its acceptable cost, stability in water and because it is commercially available in a porous-type structure such as found in Astro Met® materials (Astro Met, Cincinnati, Ohio). [0057]

EXAMPLES

Example 1

A solution from origin A, containing a total concentration of 7051 ppb of phenolic contaminant compounds, was treated in a reactor built with antimony-doped tin dioxide anodes for 72 hours. The anodic current density was 5.6 mA/cm[0058] ², the flow rate was 8.2 l/minute, corresponding to a volume to treat of 6.8 liters of solution per volume liter of reactor, and the total applied current was 300 mA, corresponding to 76104 coulombs. After the treatment period, the final total concentration of the phenolic compounds went down to 26 ppb. The concentration decrease of each species is shown in Table 2.

TABLE 2

Compound\

charge (C) 0 4925 24077 31738 50890 76104

Phenol 2600 1500 140 63 0 0

0-cresol 51 24 0 0 0 0

m-cresol 600 270 20 10 0 0

p-cresol 3800 1100 71 44 30 26

Example 2

A solution from origin A, containing a total concentration of 7519 ppb of phenolic contaminant compounds, was treated in a reactor built with antimony-doped tin dioxide anodes for 48 hours. The anodic current density was 2.8 mA/cm[0059] ², the flow rate was 8.2 l/min., corresponding to a volume to treat of 32.8 liters of solution per volume liter of reactor, and the total applied current was 600 mA, corresponding to 77760 coulombs. After the treatment period, the final total concentration of the phenolic compounds went down to 23 ppb. The concentration decrease of each species is shown in Table 3.

TABLE 3

Compound\

charge (C) 0 5683 10543 38362 48752 77760

Phenol 2800 910 630 23 0 0

0-cresol 39 12 0 0 0 0

m-cresol 480 210 140 0 0 0

p-cresol 4200 730 220 38 29 23

Example 3

A solution from origin B, containing a total concentration of 2783 ppb of phenolic contaminant compounds, was treated in a reactor built with antimony-doped tin dioxide anodes for 12 hours. The anodic current density was 1.4 mA/cm[0060] ²,the flow rate was 8.2 l/min., corresponding to a volume to treat of 32.8 liters of solution per volume liter of reactor, and the total applied current was 150 mA, corresponding to 6480 coulombs. After the treatment period, the final total concentration of the phenolic compounds went down to 710 ppb. The concentration decrease of each species is shown in Table 4.

TABLE 4

Compound\

charge (C) 0 540 1060 3240 4860 6480

Phenol 1174 1298 1015 643 586 540

0-cresol 25 13 13 9 8 7

m− + p-cresol 1219 762 593 385 259 163

Total 2418 2073 1621 1038 853 710

Example 4

A solution from origin B, containing a total concentration of 2374 ppb of phenolic contaminant compounds, was treated in a reactor built with tantalum-doped iridium dioxide anodes for 6 hours. The anodic current density was 5.6 mA/cm[0061] ², the flow rate was 8.2 l/min., corresponding to a volume to treat of 32.8 liters of solution per volume liter of reactor, and the total applied current was 300 mA, corresponding to 6480 coulombs. After the treatment period, the final total concentration of the phenolic compounds went down to 922 ppb. The concentration decrease of each species is shown in Table 5.

TABLE 5

Compound\

charge (C) 0 540 1080 2160 4320 6480

Phenol 822 772 754 665 486 358

0-cresol 27 25 25 20 14 10

m− + p− 1525 1454 1474 1176 801 554

cresol

Total 2374 2251 2253 1861 1301 922

Example 5

A solution from origin B, containing a total concentration of 2343 ppb of phenolic contaminant compounds, was treated in a reactor built with tantalum-doped iridium dioxide anodes for 12 hours. The anodic current density was 2.8 mA/cm[0062] ², the flow rate was 8.2 l/min., corresponding to a volume to treat of 32.8 liters of solution per volume liter of reactor, and the total applied current was 150 mA, corresponding to 6480 coulombs. After the treatment period, the final total concentration of the phenolic compounds went down to 272 ppb. The concentration decrease of each species is shown in Table 6.

TABLE 6

Compound\

charge (C) 0 540 1620 3330 4860 6480

Phenol 771 600 432 272 202 116

0-cresol 28 17 14 8 8 6

m− + p− 1545 951 733 380 298 151

cresol

Total 2343 1568 1179 660 509 272

Example 6

A solution from origin B, containing a total concentration of 2783 ppb of phenolic contaminant compounds, was treated in a reactor built with tantalum-doped iridium dioxide anodes for 12 hours. The anodic current density was 1.4 mA/cm[0063] ²the flow rate was 8.2 l/min., corresponding to a volume to treat of 32.8 liters of solution per volume liter of reactor, and the total applied current was 75 mA, corresponding to 6480 coulombs. After the treatment period, the final total concentration of the phenolic compounds went down to 115 ppb. The concentration decrease of each species is shown in Table 7.

TABLE 7

Compound\

charge (C) 0 270 540 1080 1620 6480

Phenol 788 704 524 380 285 37

0-cresol 36 31 21 17 13 7

m− + p− 1960 1551 1066 723 515 70

cresol

Total 2783 2286 1612 1121 813 115

Example 7

A solution from origin B, containing a total concentration of 1369 ppb of phenolic contaminant compounds, was treated in a reactor built with anodes made of platinum electroplated on sponge titanium substrate for 9 hours. The anodic current density was 5.6 mA/cm[0064] ², the flow rate was 8.2 l/min., corresponding to a volume to treat of 32.8 liters of solution per volume liter of reactor, and the total applied current was 200 mA, corresponding to 6480 coulombs. After the treatment period, the final total concentration of the phenolic compounds went down to 48 ppb. The concentration decrease of each species is shown in Table 8.

TABLE 8

Compound\

charge (C) 0 720 1440 3120 4560 6480

Phenol 359 286 253 173 119 16

0-cresol 22 15 15 11 8 4

m− + p− 988 690 595 373 250 29

cresol

Total 1369 990 863 557 377 48

Example 8

A solution from origin B, containing a total concentration of 1994 ppb of phenolic contaminant compounds, was treated in a reactor built with anodes made of platinum electroplated on foam titanium substrate for 18 hours. The anodic current density was 2.8 mA/cm[0065] ²,the flow rate was 8.2 l/min., corresponding to a volume to treat of 32.8 liters of solution per volume liter of reactor, and the total applied current was 100 mg, corresponding to 6480 coulombs. After the treatment period, the final total concentration of the phenolic compounds went down to 49 ppb. The concentration decrease of each species is shown in Table 9.

TABLE 9

Compound\

charge (C) 0 360 1440 2880 4680 6480

Phenol 519 416 246 97 54 23

0-cresol 29 20 13 6 3 2

m− + p− 1445 1078 585 156 46 24

cresol

Total 1994 1514 844 259 103 49

Example 9

A solution from origin B, containing a total concentration of 1829 ppb of phenolic contaminant compounds, was treated in a reactor built with anodes made of platinum electroplated on foam titanium substrate for 18 hours. The anodic current density was 1.4 mA/cm[0066] ², the flow rate was 8.2 l/min., corresponding to a volume to treat of 32.8 liters of solution per volume liter of reactor, and the total applied current was 50 mA, corresponding to 6480 coulombs. After the treatment period, the final total concentration of the phenolic compounds went down to 52 ppb. The concentration decrease of each species is shown in Table 10.

TABLE 10

Compound\

charge (C) 0 360 720 1260 4320 6480

Phenol 479 375 255 137 23 29

0-cresol 24 19 13 9 3 2

m− + p− 1356 957 651 381 38 21

cresol

Total 1829 1351 919 528 64 52

Claims

We claim:

1. A flow-through electrochemical reactor comprising:

2. A flow-through electrochemical reactor according to claim 1, wherein the porous anode comprises a foam.

3. A flow-through electrochemical reactor according to claim 1, wherein the porous anode comprises a substrate having an anodic coating.

4. A flow-through electrochemical reactor according to claim 3, wherein the substrate is tantalum or titanium.

5. A flow-through electrochemical reactor according to claim 3, wherein the anodic coating is selected from the group consisting of platinum, tantalum-doped iridium dioxide and antimony-doped tin dioxide.

6. A flow-through electrochemical reactor according to claim 1, wherein the at least one cathode is a porous cathode, and wherein the at least one porous cathode is sized in the internal chamber so that the wastewater passes through the pores of the porous cathode.

7. A flow-through electrochemical reactor according to claim 6, wherein the porous cathode comprises a foam.

8. A flow-through electrochemical reactor according to claim 1, wherein the cathode comprises nickel.

9. A flow-through electrochemical reactor according to claim 1, wherein the body is tubular and the internal chamber is generally cylindrical, and wherein each anode and cathode is supported by an insulating holder sized to be slidably inserted into the internal chamber.

10. A flow-through electrochemical reactor according to claim 1, wherein the reactor comprises from two to ten anodes and from three to eleven cathodes, respectively, in alternating arrangement.

11. A flow-through electrochemical reactor according to claim 10, wherein the reactor comprises seven cathodes and six anodes.

12. A flow-through electrochemical reactor according to claim 1, wherein the target substance comprises an aryl compound, and, in use, the reactor produces an electrical current having a current density capable of oxidizing the aryl compound.

13. A flow-through electrochemical reactor according to claim 12, wherein the aryl compound is selected from the group consisting of phenol, o-cresol, m-cresol and p-cresol.

14. A flow-through electrochemical reactor according to claim 13, wherein the aryl compound is phenol.